1. Field of Art
The present invention relates to an apparatus for molding lost wax masters used in the investment casting or for molding articles of thermoplastic material through the injection molding. More particularly, it relates to an injection molding an apparatus for molding articles having hollow core portions.
2. Prior Art
Injection molding processes have been widely used not only for the production of molded articles made of various thermoplastic resins but also for the production of lost wax masters (hereinafter referred to as wax models) used in the investment casting process. Since the molded products, such as wax masters, tend to shrink during the cooling step in or after the injection molding process, each molding cavity has the dimensions slightly larger than those of the finished product molded therein. However, due to this shrinkage resulted by cooling, the surface of the wax mold or molded product is often caved to suffer so-called "sink mark" particularly when the molded product is relatively thick and has large surface area or areas. The dimensional stability of the finished product is seriously affected by the formation of "sink mark" to make it difficult to mold the wax model having accurate dimensions.
In conventional injection molding processes, the wall thickness of the wax or other thermoplastic materials is decreased as thin as possible by embedding an inlay made of another thermoplastic material or by incorporating a core within the mold cavity. However, these known methods are disadvantageous in that the producibilities thereof are poor and hence they are not suited for mass production.
In order to eliminate the need of embedding an inlay or core made of another material, an improved process has been proposed wherein a molten thermoplastic material is injected in each mold cavity and then high pressure air is fed in each mold cavity to back up the mass of plastic material during the cooling step to prevent shrinkage of molded article due to cooling. The specification of Japanese Patent Application No. 123944/1974 discloses a nozzle assembly suited for this purpose. However, the nozzle assembly disclosed by the preceding patent application is not fully satisfactory in that the volume of air flown into the mold cavity is too small since air is fed into the mold cavity previously filled with a pressurized thermoplastic material, and in that the air supply passage is frequently blocked by the cooled and solidified thermoplastic material. It is, therefore, required to increase the air pressure to a very high level in order to feed sufficient volume of air in each mold cavity.